Let's check that everything is working correctly. I don't know if Ubuntu installs another MTA but do you have any other mailserver running on your system? Run the following commands on your server:

Code:

su - zimbra
zmcontrol status

That should show all the services running, if the MTA isn't running then do this:

Code:

telnet localhost 25

If you get a valid response from that then you have another MTA running and it needs to be stopped and disabled. If you need to stop it then restart Zimbra again and see if you can send email. If there's still the same problem then run these commands again and post the output: